Glyoxal is one of the several multifunctional species that are formed in significant yields from the photooxidation of aromatic hydrocarbons, isoprene as well as other biogenic sources. Glyoxal is reactive and highly water-soluble. Recent emerging evidence indicates that these properties enable it to participate in aqueous and heterogeneous reactions to form nonvolatile adducts with existing aerosol constituents, thereby contributing to secondary organic aerosol (SOA). Given the abundance of its precursors in ambient environment, glyoxal could be an important contributor to SOA. The existing evidence from laboratory and chamber studies indicate that glyoxal partitions into the aerosol to a much higher degree than expected from its volatility. Certain parameters (e.g., Henry’s Law consta...[ Read more ]

Glyoxal is one of the several multifunctional species that are formed in significant yields from the photooxidation of aromatic hydrocarbons, isoprene as well as other biogenic sources. Glyoxal is reactive and highly water-soluble. Recent emerging evidence indicates that these properties enable it to participate in aqueous and heterogeneous reactions to form nonvolatile adducts with existing aerosol constituents, thereby contributing to secondary organic aerosol (SOA). Given the abundance of its precursors in ambient environment, glyoxal could be an important contributor to SOA. The existing evidence from laboratory and chamber studies indicate that glyoxal partitions into the aerosol to a much higher degree than expected from its volatility. Certain parameters (e.g., Henry’s Law constant) that are important for quantifying and modeling contributions of SOA contributions by glyoxal were not yet available prior to this thesis work. In addition, the partitioning behaviors of glyoxal in ambient aerosols have yet been quantified and the affecting factors to be identified through ambient measurements. In this thesis work, partitioning mechanisms of glyoxal in atmosphere has been investigated through laboratory and field studies.

Henry’s law constant of glyoxal in pure water was first studied in different temperatures by a bubble column technique. Along with glyoxal, glycolic acid and glyoxylic acid were also determined because of their potential role in SOA formation. Our results indicated that Henry’s law constant of glyoxal in pure water alone could not explain the level of particle-phase glyoxal that is typically found in polluted region. The effects of acidity, ionic strength and sulfate on Henry’s law constants was then determined under a variety of experimental conditions. The measurement results indicate that sulfate has a much more prominent effect on the effective Henry’s law constant of glyoxal than acidity and ionic strength. The Henry’s law constant of glyoxal can be enhanced by a few orders of magnitude in the presence of sulfate, compared to that of pure water. The enhancement of glyoxal Henry’s law constant in the presence of sulfate suggests that sulfate-mediated reactions are important in glyoxal partitioning between gas and aqueous phase.

An annular denuder coated with sodium sulfite coupled with a filter was developed for simultaneously collecting gas and aerosol phase carbonyl compounds. The newly developed method was then employed in a field study at Tsuen Wan Site in the summer of 2009 and in the winter of 2010. The average fraction of glyoxal in the particle phase was measured to be 0.029 in the summer samples and 0.035 in the winter samples. Using the measured gas-particle partitioning data and assuming the aerosol aqueous phase as the effective sorbate for glyoxal, we calculated that an effective Henry’s law constant (K_H) in the order of 10⁹ M atm^-1 would be required to explain the fraction of glyoxal observed in the particle phase. This value is four orders of magnitude higher than the effective K_H measured in pure water, but is consistent with those measured in acidic sulfate solutions.

The size distribution characteristics of glyoxal, together with methylglyoxal, glycolaldehyde, glyoxylic acid and pyruvic acid, were examined using 11 sets of size-segregated samples in the size range of 0.056-18 μm. The samples were collected on the campus of HKUST during November 2008. The size distribution of glyoxal was characterized by a dominant droplet mode with a mass median aerodynamic diameter (MMAD) in the range of 0.97-1.20 μm, accounting for ~40% of the total glyoxal mass. The proportion of glyoxal in the condensation mode (29%, MMAD: 0.15-0.42 μm) and the coarse mode (32%, MMAD: 4.2-5.8 μm) were smaller. Link was found between liquid water content / sulfate and selected carbonyl concentrations in particles. This link indicated that these species are actively involved in aqueous phase reactions. The size distribution measurement work supports the hypothesis that sulfate plays an important role in retaining glyoxal in the aerosol phase.

Much additional work, both laboratory-oriented and field-based work, is needed in order to improve our understanding of the overall gas/particle partitioning mechanisms of glyoxal. Expansion of similar work to a few other C2 and C3 bifunctional compounds is also needed to quantify their contributions to SOA.